The photovoltaic industry (PV) industry is growing rapidly and is responsible for an increasing amount of silicon being consumed beyond the more traditional uses as integrated circuit (IC) applications. Today, the silicon needs of the solar cell industry are starting to compete with the silicon needs of the IC industry. With present manufacturing technologies, both integrated circuit (IC) and solar cell industries require a refined, purified, silicon feedstock as a starting material.
Materials alternatives for solar cells range from single-crystal, electronic-grade (EG) silicon to relatively dirty, metallurgical-grade (MG) silicon. EG silicon yields solar cells having efficiencies close to the theoretical limit, but at a prohibitive price. On the other hand, MG silicon typically fails to produce working solar cells. Early solar cells using polycrystalline silicon achieved very low efficiencies of approximately 6%. In this context, efficiency is a measure of the fraction of the energy incident upon the cell to that collected and converted into electric current. However, there may be other semiconductor materials that could be useful for solar cell fabrication. In practice, however, nearly 90% of commercial solar cells are made of crystalline silicon.
Cells commercially available today at 24% efficiencies are made possible by higher purity materials and improved processing techniques. These engineering advances have helped the industry approach the theoretical limit for single junction silicon solar cell efficiencies of 31%.
Because of the high cost and complex processing requirements of obtaining and using highly pure silicon feedstock and the competing demand from the IC industry, silicon needs usable for solar cells are not likely to be satisfied by either EG, MG, or other silicon producers using known processing techniques. As long as this unsatisfactory situation persists, economical solar cells for large-scale electrical energy production may not be attainable.
Several factors determine the quality of raw silicon material that may be useful for solar cell fabrication. Silicon feedstock quality often fluctuates depending on the amount of impurities present in the material. The main elements to be controlled and removed to improve silicon feedstock quality are boron (B), phosphorous (P), and aluminum (Al) because they significantly affect the resistivity of the silicon. Feedstock silicon materials based on upgraded metallurgical (UM) silicon very often contain similar amounts of boron and phosphorous. And while chemical analysis may be used to determine the concentrations of certain elements, this approach requires too small of a sample size (a few grams) and often provides variable results—for example, the amount of boron present may vary from 0.5 parts per million by weight (ppmw) to 1 ppmw. Further, chemical analysis on different batches have provided consistent boron and phosphorous concentrations but with extreme variation in electrical parameters. These unreliable results may be due to the large affects relatively minor impurities produce.
Resistivity is one of the most important properties of silicon (Si) used for manufacturing solar cells. This is because solar cell efficiency sensitively depends on the resistivity. State-of-the-art solar cell technologies typically require resistivity values ranging between 0.5 Ω·cm and 5.0 Ω·cm. Currently produced feedstock materials based on UM silicon often come with a base resistivity below the minimum resistivity of 0.5 Ω·cm that is typically specified by solar cell manufacturers. There is a simple reason for this: Expensive processes for upgrading UM-Si are primarily concerned with taking out non-metals, including dopant atoms B and P. In order to reduce cost, there is a clear tendency to minimize such processing, i.e., UM-Si typically still contains high concentrations of dopant atoms.
Purification by segregation during directional solidification is often used in the process to obtain upgraded metallurgical silicon. Impurity removal methods include directional solidification which concentrates impurities such as B, P, Al, C, and transition metals in the last part of the resulting silicon ingot to crystallize—often the top of the ingot. In a perfect case, the crystallization during the directional solidification process would be uniform from top to bottom and the solid-liquid interface would be planar throughout the entire ingot. This would result in consistent impurity concentrations profiles from top to bottom throughout the ingot—allowing impurities in the ingot to be removed according to one flat cut across the ingot which removes top part of the ingot.
However, controlling the thermal field during a directional solidification process is difficult and often results in an inhomogeneous growth of the crystals in the silicon ingot. This causes uneven top to bottom impurity concentration profiles throughout the ingot (i.e. from one end of the ingot to another). This effect is further amplified in mass production of large amounts of silicon. Because different areas of the ingot have different impurity profiles, and thus different resistivity profiles, a flat cut across the ingot does not maximize the usable silicon yield while still removing most of the concentrated impurities.
Further, variability in incoming UMG-Si feedstock quality necessitates control process for testing and analyzing UMG-Si material quality. Typically, elements such as boron (B) and phosphorous (P) can degrade Si feedstock quality. If not controlled within certain concentration limits they produce sizable variations in ingot resistivity. Other elements such as, but not limited to, carbon, oxygen, nitrogen, compounds having these elements, in particular SiC, may also degrade ingot quality.
Due to the large effects of these and similar impurities, feedstock materials should be analyzed and tested to ensure proper quality. Batch to batch variations in the impurities and resistivity of incoming feedstock affect bottom to top resistivity of ingots and yield (n-type part vs. p-type part).
Suppliers of UMG-Si feedstock may not rigorously establish quality control of the materials they ship to their customers. Often, typical chemical analyzes produces unreliable results because of the large effects relatively minor impurities produce. Further, suppliers often test too small a sample size in relation to the variability of boron and phosphorous concentrations in the feedstock batch. Additionally, superimposed measurement error makes measurement results uncertain. One indication of these measurement errors occurs when chemical analysis on different batches yields the same boron and phosphorous content despite variation in electrical parameters. For companies relying on a plurality of UMG-Si feedstock batches to cast silicon ingots, these variations among batches may not be acceptable.